1. Field of the Invention
The present invention relates to an optical modulator of Mach-Zehnder type (branch-and-interfere type) with a monitoring function of the output light.
2. Description of the Related Art
In order to increase transmission capacity of an optical communication system using a quartz optical fiber as the optical transmission line, it is effective to use a light beam with a wavelength of 1.3 .mu.m at which the wavelength dispersion hardly occurs. On the other hand, in order to increase the repeater-to-repeater distance in the system as described above, it is effective to use a light beam with a wavelength of 1.55 .mu.m at which the transmission loss is held down to the minimum.
Therefore, in order to increase the transmission capacity and, further, to extend the repeater-to-repeater distance in optical communication systems, it becomes necessary to use a light beam with a wavelength of 1.55 .mu.m and to avert the effect of the wavelength dispersion by some means. As one of the arts to avert the effect of the wavelength dispersion, there is an external modulation system in which an external optical modulator is provided independently of a laser diode and laser beam from the laser diode, which constantly emits laser beam, is indirectly intensity modulated by the modulator.
Since, according to this system, the laser diode can be driven under constant conditions, it becomes possible to stabilize the lased wavelength and avert the effect of the wavelength dispersion. As one of the optical modulators to be used in practicing the external modulation system, there is a Mach-Zehnder (branch-and-interfere type) optical modulator using optical waveguides. There are demands for the optical modulators of the described type that are improved in reliability and made smaller in size.
FIG. 1 is a plan view of a prior art Mach-Zehnder optical modulator module with a monitor. On a planar waveguide substrate 10 made of LiNbO.sub.3 (lithium niobate), there are formed an input-side optical waveguide 12, branch optical waveguides 14a and 14b, and an output-side optical waveguide 16 by thermal diffusion, for example, of Ti (titanium) into the substrate. The branch optical waveguide 14a is provided with a grounding electrode 18 mounted thereon and the branch optical waveguide 14b is provided with an electrode 20 for a progressive wave mounted thereon.
The input terminal 22 of the progressive-wave electrode 20 is adapted to be supplied with a modulating high-frequency signal and the output terminal 24 is provided with a terminating arrangement. With the described arrangement, when the modulating signal is input, the branched beams being in phase when divided can be given different phase changes. There are provided reflection preventing films 26 and 28 for the input end face and the output end face of the waveguide substrate 10, respectively. The waveguide substrate 10 is attached onto a board 25 for the modulator module.
Referring to FIG. 2, the output-side optical waveguide 16 is provided with an optical coupler 30 arranged by having one end portion of an optical waveguide 32 for taking out monitor light disposed close to the output-side optical waveguide 16, and the monitor light taken out by means of the optical coupler 30 is detected by a photodiode (PD) 34 provided at the other end portion of the optical waveguide 32 and converted into an electric signal.
Referring again to FIG. 1, the electric signal from the PD 34 is fed back to a modulating signal driver circuit, not shown, through terminals 36 and 38, whereby a DC bias voltage applied between the electrodes 18 and 20 is adjusted. Emitted light from a constant-polarization fiber 40 fitted to a ferrule 42 is condensed by lenses 44 and 46 and coupled to the input-side optical waveguide 12. On the other hand, the intensity modulated light emitted from the output-side optical waveguide 16 is condensed by lenses 47 and 48 and coupled to a single-mode optical fiber 50 fitted to a ferrule 52.
In operation, light emitted from the constant polarization fiber 40 is coupled to the input-side optical waveguide 12 through the lenses 44 and 46 and propagated through the branch optical waveguides 14a and 14b to be combined again in the output-side optical waveguide 16. Since the input-side optical waveguide 12 and output-side optical waveguide 16 are arranged to be single-mode optical waveguides propagating only light of the basic mode, the intensity of the output interference light is maximized when the phase difference between the branched light beams is zero and the intensity of the interference light is minimized when the phase difference is .pi.. When the phase difference is between zero and .pi., the interference intensity takes on a value corresponding to the phase difference. Thus, intensity modulation of light corresponding to a modulating signal can be achieved.
Now, in the prior art optical modulator module as shown in FIG. 1 and FIG. 2, it has not been possible to reduce the radius of curvature R of the waveguide 32 for taking out monitor light so much, in view of the loss in the waveguide 32. Further, since the signal light and monitor light have been taken out in the same direction, the waveguide 32 for taking out the monitor light have had to be separated from the output-side optical waveguide 16 by a distance allowing the PD 34 to be attached to the end face of the waveguide substrate 10. Thus, there has been a disadvantage that the length of the waveguide substrate 10 becomes as long as approximately 70-80 mm.
Further, since the output end face of the waveguide substrate 10 has been formed to be perpendicular to the output-side optical waveguide 16, it has been necessary to provide the reflection preventing film 28 on the output end face of the waveguide substrate 10, in order to avert the harmful effect of the reflected light from the output end face of the waveguide substrate 10. Furthermore, when assembling the module, it has been necessary to bring the plane of polarization of the input light from the constant polarization fiber 40 into a specified relative position to the optic axis of the waveguide substrate 10 and, hence, there has been a problem that the power of the monitor light varies when the plane of polarization is misaligned in the module assembling.